What does a mustard seed grow into?
The Parable of the Mustard Seed
31 "He put before them another parable: “The kingdom of heaven is like
a mustard seed that someone took and sowed in his field; 32 it is the
smallest of all the seeds, but when it has grown it is the greatest of
shrubs and becomes a tree, so that the birds of the air come and make
nests in its branches.”
The mustard seed is one of the tiniest seeds and in the parable represents "The Kingdom of Heaven", the sowing of it, represents the preaching of the good news .
(Luke 4:43 (NRSV)
43 "But he said to them, “I must proclaim the good news of the kingdom
of God to the other cities also; for I was sent for this purpose.”
The Christian congregation had its tiniest beginnings in 33 C.E., but within a few decades the growth was rapid and the congregation grew to such an extend, that Paul was able to say, that the gospel was proclaimed to every creature under heaven.
Colossians 1:23 (NRSV)
23 "Provided that you continue securely established and steadfast in
the faith, without shifting from the hope promised by the gospel that
you heard, which has been proclaimed to every creature under heaven.
I, Paul, became a servant of this gospel."
The birds represent persons ,that accepted the gospel or spiritual food, and find shelter and protection within the Christian congregation.
Regarding the Messianic King , Jesus, Ezekiel wrote.
Ezekiel 17:23 Amplified Bible (AMP)
23 "I will plant it on the mountain heights of Israel, that it may
grow boughs and bear fruit and be a noble and stately cedar. And birds
of every kind will live under it; they will nest [securely] in the
shade of its branches."